At about 6:10 p.m. Friday a Metro-North train heading east from New York City derailed outside Bridgeport, Conn., and was hit by a westbound train on an adjacent track. Some 700 passengers were on board the trains, the Associated Press reported.

The cause was unclear. Some local officials said it did not appear to be terrorism, and a team from the National Transportation Safety Board arrived Saturday to investigate issues ranging from crew performance to the condition of the track.

Commuter rail accidents are relatively rare. One federal study, looking at the decade between 1996 and 2005, found a declining rate of accidents (with 33 accidents or incidents per million total commuter-train miles in 2005).

Injuries generally totaled fewer than 2,000 per year, and fatalities fewer than 100 per year, during that 10-year period.